Most conventional Integrated Circuit (IC) fabrication process steps can be characterized as either "front end" steps or "back end" steps. Front end steps generally include those steps necessary to form the actual transistor elements such as source/drain regions, gates, and isolation regions. Back end steps generally include those steps necessary to create circuitry by wiring the various transistors formed by the front end processing. The integrated electronic circuitry created in the back end steps includes complex line routing patterns (or "wiring") between transistors at the integrated circuit substrate level. The wiring is provided as conductive vertical interconnects and patterned horizontal metallization layers sitting in a layered stack above the substrate. To electrically insulate the metallization layers from one another and from the semiconductor wafer or silicon substrate or dielectric layers are sequentially deposited atop a crystalline wafer or chip using separate coating techniques.
One common coating technique employs high-speed centrifugal spinning of silicon wafers to apply Spin-On Dielectrics (SODs) or resist coatings during fabrication. Resists are photosensitive films which are used to generate patterns over films when they are exposed to an appropriate light source. SOD's, on the other hand, are spin-on-dielectric films. Typically, to form about a 1 .mu.m film on an eight (8) inch substrate, at least about three (3) to about five (5) ml of a SOD or resist fluid is sprayed, streamed or dribbled onto the substrate surface. Briefly, these fluids will heretofore be referred to as "precursor fluids" which represent the particular fluids employed as a precursor to particular film layer formation during integrated circuit fabrication. After deposition of the precursor fluid, the silicon substrate is accelerated at 10,000 to 20,000 RPM/sec and spun about a rotational axis at final speeds ranging from about 2000 rpm to about 6000 rpm. Along with a plurality of other parameters, the acceleration and spin speed during the spin cycle cooperate to apply a thin, uniformly distributed film coating on the substrate.
While this "spin coating" technology is well entrenched in the integrated chip fabrication industry as the primary method of fluid deposition, several problems are inherent with this technique. The centrifugal force imposed upon the viscous precursor fluids during the spin process in many cases cause, the loss of over ninety-nine percent (99%) of the fluid from the substrate surface. Accordingly, about 99% of the precursor fluid which is disposed of in a spin cup remains unused, and is subsequently wasted. Considering the fact that the precursor fluid cost ranges from about one dollar ($1) to about three ($3) per gram, or up to about four thousand dollars ($4000) per gallon, the amount of wasted material in this process is substantial.
Another problem associated with the prior art spin coating technology is the generation of particulates or fluid droplets which are ejected from the wafer edge during the "spiral" stage. These droplets, which travel off the wafer at high speeds, are a possible source of splashback or redeposition onto the wafer surface. Due to the abundance of precursor fluid ejected into the waste bowl during the spin cycle, costly disposal problems may occur if the fluid is environmentally unfriendly and cannot be reused. Moreover, the bowled or closed-cup spin trays enclosing the spun wafer require frequent internal washing due to impingement and coating of the internal components with the spinoff fluid therewith.
Meniscus-type coating is another coating technique commonly applied to generate resist and dielectric films on wafers. In this process, a wafer carrier mechanism is provided which includes a plate member having a plate surface adapted to support the wafer thereon. This planar plate surface defines a circular receptacle formed and dimensioned to securely seat and support the circular silicon substrate therein such that the planar substrate surface is seated flush with the planar plate surface. An elongated coating head device is provided which is oriented and adjusted just above the substrate/wafer carrier unit. To form the meniscus coating atop the substrate surface, the coating head sweeps linearly across the substrate surface and the plate surface, simultaneously depositing and distributing a meniscus coating of precursor fluid thereon.
While this technique comparatively reduces precursor fluid waste, an appreciable amount of material is still wasted due to the formation of a film on the plate surface surrounding the wafer. Such film formation on the plate surface, however, is necessary to reduce backside deposition of the fluid at the peripheral edge of the circular silicon substrate which would occur if the coating head only swept across the circular substrate surface alone. By providing a flush surface substantially juxtaposed to the peripheral edge of the seated,substrate (i.e., the plate surface of the plate member), backside deposition of fluid can be substantially reduced or eliminated.
Unfortunately, coating of the substrate surface peripheral edge and the plate surface receptacle edge occurs due to the inevitable deposition of precursor fluid in the seam therebetween. Once the meniscus coating has cured to form the resist or dielectric film layer, the substrate must be separated from the plate member. Hence, this seam must be broken during separation which causes unpredictable results such a chipping and/or delamination. Moreover, these problems may lead to particulate formation which can damage devices in subsequent process steps.
Finally, in this meniscus coating process, the film thickness is essentially determined by the meniscus thickness. These films, thus, tend to be relatively thick on the order of between about 2 .mu.m to about 10 .mu.m. Comparatively, the spin-coating technique typically yields films on the order of about 1 .mu.m or less.